Help with my latest bad idea...building a bike!

So the last thread I started I wanted a new set of wheels to more quickly change from road to trail tires on my only bike. You guys gave me some excellent and highly sensible advice...which I promptly ignored..
So I bought a new set of wheels (black flag experts ebay $170) and noticed my parts bin filling up with barely used take offs...and then my next bad idea hit me...build a new bike for neighborhood riding!

The new bike will be for toolin around and exercise in the neighborhood. I would like to be able to put knobbies on it and loan it to a friend for light trail use as well.

I realize this makes no practical or financial sense, and there are much cheaper and faster ways to get a bike for this purpose, but i really enjoy wrenching and learning in the process...so no trying to talk me out of it.

What I need is frame, complete drivetrain and brakes.
I have myself on a strict monthly hobby budget so will try to piece it together over a few months with ebay or used parts. So i have some questions which I haven't been able to answer completely with searching.

Drivetrain
This is where my head is spinning. I'd like to upgrade my trail bike (Airborne Bicycles. Skyhawk) and swap those parts onto the new bike. Looking at the SRAM x7 stuff. I wanted to do 2x10 but there seems to be less inexpensive options, especially for the crankset.
So questions:

1.Bottom bracket. Both frames say 68mm english. I should just be looking at cranksets that are "standard BB"? I don't want any bb30 or bb90 right?

2.Deraileurs. Hi or low mount? long cage or short cage? I haven't found any guides that really spell it out for my simple mind. Any simple ways to measure for me to figure this out? Is it based on the gearing I choose or the frame?

3. Cassette. If i want 10 speed does my rear hub have to be a certain size? Pretty sure it is 135mm IIRC.

4.Crankset. Any advantages disadvantages of a 2 vs. 3 front set up. I am a very new rider so I have no personal preference.

Brakes
Thinking about something in the Avid BB line.

Thanks for any guidance

Mountain biking is like fishing, golf, and sex...you don't have to be good at it to have a great time.

2) Long cage vs Short cage is simple enough - if you're rolling a 3x10 you'll want a longcage. A 1x10 you'd be fine with short or medium - personally, I'd say 1x10 is fine for most people (11-36 cassette with a 32 tooth ring will climb most things just fine)

3) 135mm is the standard rear spacing for mountain bikes (with some downhillers being 150). A hub that will take an 8 or 9 speed cassette will equally take a 10 or 11 - unless its specified otherwise, you'll almost certainly be fine.

4) Personally, I prefer 1*x. The front mech is a part I find irritating and unless you're climbing something VERY steep (in which case you might want a granny ring) or really powering along on roads, the chances are you'd be fine with it -
I'm still rolling 1x9 (32 tooth front, 11-34 cass) and find very little I can't clear on that - with an extra 2 teeth on the cassette (10 speeds often go 11-36) you'd have an extra low gear (or if you go for a 34 tooth chainring, an extra high gear).
You'll probably want a jumpstop (a chaindevice would probably be overkill for you) which won't cost an awful lot, certainly less than a front mech and shifter would cost.

As far as brakes go - hydraulics generally feel nicer and will self-adjust as pads wear. Mechanicals require less special tools to work on (ie: no bleed kits) and will need trimming as pads wear (or cables stretch) but are still considerably better than rimbrakes.
I have a set of hayes mx4's sat in the spare parts bin as an emergency brakeset (ie: if I need a pair fast and dont have time to bleed before a ride - they don't feel as nice as any of my hydro's, but they're reliable.
Mechanical brakes are much of the same - bb7's have an avid following (pun intended), but there's nothing wrong with a bb5, a hayes mx1 or mx4 or a tektro unit.

Hadn't given much thought to a 1x. Pretty flat here in Florida. I know I wouldn't miss the big ring, but worry about losing the granny. I'm in the middle ring now 98% on trails but I use granny on a hill or two at the trail I most frequent. But probably more because of a lack of fitness than because of the hill itself.

I would also suggest a 1*9 or 10 setup. Unless your local trails and roads require otherwise, you're better off with less stuff on your bike, i.e., derailleur, shifter, and cable. On my bike, I find my self using the middle ring almost %98 of the time, even at the hilly sections in nearby trails. Good luck, and have fun.

Hadn't given much thought to a 1x. Pretty flat here in Florida. I know I wouldn't miss the big ring, but worry about losing the granny. I'm in the middle ring now 98% on trails but I use granny on a hill or two at the trail I most frequent. But probably more because of a lack of fitness than because of the hill itself.

When you say you sometimes use the granny ring sometimes with your present setup, do you know what size middle ring and what cassette you're using right now?
I ask since if you're using a 32 or 34 tooth ring with (say) an 11-32 cassette, you'll probably find up finding that the extra couple of low gears on an 11-36 will make enough difference that you flat-out DONT need the inner chainring anymore.
(Bear in mind also that other things will affect it a little - a lighter bike or one with faster rolling tyres you'll pull slightly higher gears, inverse also being true).